Bones make objects scale and shrink
This is (probably) because your mesh isn't correctly assigned to your rig.

Each vertex needs to know which bone(s) to follow, and how much. If you've got one vertex assigned to two bones, then that vertex will do a combination of what both bones are doing. If you've got a vertex that's only assigned to one bone, but only at 50%, then it'll only move/rotate/scale half as much as the bone does.

To check this, go into pose mode and then from there directly into weight paint mode and check the colors of the mesh as you select each bone. Blue is zero, red is one. Zero means that vertex will not follow the selected bone at all. Red means it'll follow that bone 100%. Somewhere in between will do some combination.

You can use the painting tools to modify this, but they are also just vertex groups. You can use normal vertex group assignments to assign vertices to a given bone. With a mesh as simple as yours, that might actually be easier.

Hope that helps!